Arsenate inhibition on kinetic characteristics of alkaline phosphatase as influenced by pH

pH plays an important role on the arsenic availability to biota and enzyme activities in soil. A simulation experiment was carried out to investigate the effect of pH on the kinetic characteristics of three forms of alkaline phosphatase (free, immobilized by montmorillonite and soil) under arsenate contamination at four pH levels ranging between 7 and 10. The results showed as a function of pH, high pH favored low substrate affinity to enzyme and high maximum reaction velocity under arsenate exposure at all tested concentration ranges. Though pH could affect the kinetic parameters, the arsenate inhibition mechanisms on alkaline phosphatase (ALP) were not altered. For free ALP, a linear mixed inhibition dominated by competitive effect was displayed by arsenate. For both immobilized ALP, arsenate inhibited the enzyme activity by competing for active sites with the substrate. Based on the analysis of Michaelis constant (K-m) and inhibition constant (K-ic), it was shown that arsenate always exhibited stronger binding ability on enzyme than the substrate. Moreover, as the arsenate concentration increased, the ecological dose (ED10) and K-ic raised in the fixed ALP, which indicates the immobilization by minerals has protected the ALP from arsenate toxicity. Our study reveals the important role of pH in the evaluation of heavy metal toxicity potentially employing soil enzyme as an indicator.